1. Field of the Invention
The present invention relates to an illuminating device for illuminating non-self light emission display elements and a display device used for electronic devices. In particular, the present invention relates to a liquid crystal display device used for a portable information device, a mobile telephone, and the like and an illuminating device used as a frontlight unit or a backlight unit to illuminate elements.
2. Description of the Related Art
In recent years, most of display devices used for a mobile telephone, a mobile computer, and the like are liquid crystal display devices capable of obtaining a high-definition color image with reduced power consumption. A liquid crystal element used for the liquid crystal display device is a non-self light emission type, so the liquid crystal element is illuminated by an illuminating device using a high-intensity white LED as a light source.
In particular, used for the mobile telephone is a reflection type liquid crystal display device, which has a large aperture and delivers bright images, or a liquid crystal display device of a double side visible type, which can display image information on both front and rear screens. The white LED used to illuminate a display element of each of the liquid crystal display devices has a structure in which a resin into which a green phosphor or a yellow phosphor is dispersed is provided immediately in front of a light emitting surface of a blue LED made of InGaN, GaN, or the like. According to the structure, green light or yellow light can be mixed with original blue light to obtain white light. An yttrium aluminum garnet (YAG) phosphor in which YAG is doped with a rare-earth element has been widely known as a phosphor for converting the blue light into the yellow light. In addition, a chalcogenide phosphor doped with a rare-earth element has been widely known as a phosphor for converting the blue light into the green light or red light with relatively high efficiency. For example, disclosed in JP 11-121802 A is an LED display device in which a plurality of light emitting elements, each of which emits light having a wavelength equal to or shorter than the wavelength of the blue light, are arranged on a printed circuit board having an arbitrary shape and an arbitrary area in which circuits are formed and the respective light emitting elements are coated with a translucent resin containing a wavelength conversion material.
However, in the case of additive mixture of two colors using the blue LED and the YAG phosphor (pseudo white LED), the amount of light component having a wavelength region of 600 nm or more is small. Therefore, the realization of an LCD module having high color reproducibility is inhibited. In general, when the pseudo white LED is used as a light source, it is very difficult to exceed an NTSC ratio of 100% with a current color filter technique.
On the other hand, when additive mixture of three colors using two kinds of phosphors for converting blue light into green light and red light, respectively, based on blue excitation and the blue LED (three-wavelength white LED) is used, the LCD module having high color reproducibility can be realized. However, it is the fact that a material having very high efficiency is not found as a phosphor for emitting red light based on the blue excitation. Therefore, the intensity efficiency obtained by the structure is much lower than the efficiency of the pseudo white LED and is only approximately half. Further, a chromatic variation of the three-wavelength white LED is very large, so the color balance cannot be adjusted after the assembly of an LED package. Thus, even when respective products (display devices) are composed of display panels, each of which has the same specification, it is impossible to avoid the occurrence of a chromatic difference between the respective products.